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Friction on snow surface, especially in relation to winter sports, is difficult to measure directly because of its complexity and variability. In this work, a wearable snow friction measurement device for cross-country skiing was designed and constructed to calculate the friction force and monitor movement signals through the logical placement of three-component sensors and connection parts on skis. Both theoretical simulation and experiments have been conducted, which proved that the deformation of cross-country skis outputs few abnormal force signals during applying large load. Through the static validation in the laboratory, the output accuracy and stability of the device were confirmed. To verify the availability of the device, measurements on a skiing treadmill were conducted, and the results showed that the device can work well and distinguish the different friction behaviors of various waxes. More important, through the ski field test, the device was able to monitor the technical actions and evaluate the frictional coefficient of different ski waxes on snow surface comprehensively, which provides a scientific reference for athletes and coaches in routine training. [ABSTRACT FROM AUTHOR]

High static friction between functional surfaces poses a serious challenge for industrial valves, particularly for those used in the advanced ultra-supercritical power generation industry that requires raising the steam temperature to 760 °C. In this context, we established the static friction measurement methodology by evaluating the effects of temperature, loading time, and number of static cycles on the evolution of the static coefficient of friction up to 800 °C. This methodology was applied on a nickel-based solid lubricant coating that appears an attractive candidate to withstand very harsh environments. The tribological behavior is explained in terms of a competition of three mechanisms governing the static friction processes: micro-welding or sintering of asperities, creep of asperities, and formation of lubricious oxides. • Establishment of the static friction measurement methodology to simulate valve applications. • First time to systematically report the evolution of static COF with number of static test cycles, time and temperature. • Three main mechanisms governing the static friction processes. [ABSTRACT FROM AUTHOR]

An analytical EHL model suitable for highly loaded point contacts operating in mixed and boundary regimes of lubrication is presented. The results are compared and validated with the experimental measurements of friction coefficient from an MTM tribometer. Measurements are carried out upon a PAO40 oil at medium to high temperatures. In general, a good agreement in trend and magnitude over a wide range of operating conditions is observed. Both the experimental findings and the model predictions show an inverse relationship between friction and contact load in mixed and boundary regimes of lubrication, an effect not hitherto reported in literature. [ABSTRACT FROM AUTHOR]

This work investigates the mechanisms for friction reduction in textured journal bearings. The measured friction of laser-etched connecting-rod big-end shells was compared with a non-textured reference under engine-like conditions. Experiments covered several working conditions, and the friction and lubricant film temperatures were measured for three texture configurations. The origin of the verified friction reduction was explored using a numerical model that simulates the hydrodynamic lubrication and global thermal effects. The simulation results accurately matched the friction measurements, revealing that the microscopic mechanisms of reduction in lubricant shear stress and effective viscosity due to texture-induced cavitation act simultaneously to reduce the friction coefficient for the textured shells. The findings have practical applications for the optimal design of textured hydrodynamic bearings. • Friction in textured journal bearings was experimental and numerically investigated. • A journal bearing model with thermal effects was validated using experimental data. • Textures reduced the lubricant shear stress and viscosity and lowered friction. • The shear area reduction mechanism for textured hydrodynamic bearings was confirmed. [ABSTRACT FROM AUTHOR]

Real contact area is a challenging phenomenon in tribology, where despite established theoretical models, detailed experimental studies are still lacking. This work is focused on the evolution of the real contact area of multi-asperity steel contacts during sliding, by using sub-micron characterisation of entire multi-asperity contact area and simultaneous measurement of the friction force. Steel-sapphire contact was monitored by using an in-situ optical system in real time during 5 mm of unidirectional sliding in surface roughness range R a = 0.1–1.0 µm and normal loads up to the nominal contact pressure equal to steel yield strength (Y). The results showed that the initial real contact area caused by static loading increased during sliding. At low nominal contact pressure of 0.3 Y , the increase in real contact area was the largest: for R a = 0.1 µm it went from 9.5% of the nominal area under static conditions to 15.0% after sliding, a 56% relative increase, and for R a = 1.0 µm it went from 5.0% to 11.5% after sliding, a 106% increase. The increase in real contact area was smaller at higher nominal contact pressures of 0.6 Y and 1.0 Y. At 1.0 Y for R a = 0.1 µm the real contact area went from 18.0% to 21.8% after sliding and for R a = 1.0 µm it went from 9.4% to 12.9% after sliding. The coefficient of friction increased with roughness at the lowest nominal contact pressure 0.3 Y (for R a = 0.1 µm it was 0.24 and for R a = 1.0 µm it was 0.29) but became independent of roughness at higher contact pressures (on average 0.18). • In-situ measurement of real contact area evolution during 5 mm of sliding. • Whole multi-asperity steel contacts observed using optical method. • Real contact area increased due to sliding from the initial static conditions. • The largest increase from static to sliding conditions was at the lowest load. [ABSTRACT FROM AUTHOR]

The (cyclopentadienyl)molybdenum(dicarbonyl)(nitrosyl) complex as a metal-organic precursor and acetylene gas were used to produce the MoC x /a-C:H film by the PECVD. When the acetylene gas is mixed with a vaporized metal-organic precursor, the friction coefficient and hardness would be superior for the MoC x /a-C:H film, whereas the specific resistivity slightly high. The friction coefficient measurements by the ball-on-disk wear tester dramatically reduced from 0.25 to 0.06 as the acetylene flow rate increased. The electrical specific resistivity slightly high from 5.74 mΩ-cm to 203.32 mΩ-cm following the increasing acetylene partial pressure. XPS analysis indicated that the Mo-C bond in the Mo3d peaks diminished, and the sp3/sp2 ratio in the C1s peaks reached 0.37 in the condition of acetylene gas addition. [ABSTRACT FROM AUTHOR]

The methodology for measuring the frictional force of granular materials of plant origin is complex, since it requires taking into account both variable material factors and the use of available measurement methods in an appropriate way. This article deals with one of the important aspects of the application of a measurement methodology using bottomless containers. The effect of grain pressure on the side walls of the measuring container on the value of grain pressure on the measuring surface was studied. Five different granular materials (wheat, oat, beans, maize, peas) were used as the research material. To analyse the problem, the Janssen model was used as a basis, which describes the normal pressure of the granular material at the bottom of the container depending on certain factors. The tests were carried out for different heights of the material layer and different load values. After analysing the measurement values with calculated values, modifications of the base model were proposed. This modification allowed a high (close to 1) model fit to the measured values. The proposed approach can be useful in carrying out measurements of the friction force of granular materials, since it allows one to estimate the value of the actual pressure of the material on the contact surface. This provides an opportunity to make better use of the horizontal tribometer with a measuring container with support. [Display omitted] • Methodological problems of friction measurement of granular materials are discussed. • The effect of lateral grain pressure on vertical pressure on the surface is shown. • The possibility of using the Janssen model for small containers was studied. [ABSTRACT FROM AUTHOR]

Work was carried out to generate wheel/rail interface creep force data with the presence of leaf contamination. To enable this. the HAROLD full-scale wheel/rail test rig was upgraded to give friction measurement capability and methods for creating leaf layers were also developed. A unique dataset has been created not previously available for full-scale test conditions and leaf layers. The work has shown the importance of the shear induced in the brake tests for creating the black, well-bonded leaf layer. It was found in the tests that ultra-low adhesion was achieved in all tests with leaves regardless of load applied and speed. The friction was also low for a number of braking events, even when the layer had been partially removed. [ABSTRACT FROM AUTHOR]

Abstract The ultra-high molecular weight polyethylene (UHMWPE) has become one of the most popular and reliable material for water-lubricated bearings in recent years. In this study, three kinds of new Koch snowflake textures were designed on the surface of the UHMWPE to try to decrease friction and prolong the service life of the bearings. The wear process of the UHMWPE/QSn7-0.2 rubbing pair was analyzed using a specially designed tester under water-lubricated conditions. The analysis results demonstrated the testing velocity and load significantly affected the friction coefficient of the UHMWPE specimens. The UHMWPE specimen without texture-treatment had higher friction coefficient than that of the textured UHMWPE specimens at high velocity, where the maximum rate of increase was 35.2% at the velocity of 350r/min compared with non-texture. The dominant wear mechanism was abrasive wear and slight adhesive wear for the non-texture UHMWPE sample, and the wear mechanism of textured samples was slight abrasive wear. The wear performance of the UHMWPE with composite texture was superior to that of other textures, which was mainly attributed to the micro hydrodynamic bearing effect and stepped bearing effect. As a result, the findings of this work can provide experimental foundation for texture design and optimization on UHMWPE surfaces for better service of the marine water-lubricated bearings. Highlights • The UHMWPE specimen without texture-treatment had higher friction coefficient than that of the textured UHMWPE specimens at high velocity, where the maximum rate of increase was 35.2% at the velocity of 350r/min compared with non-texture. • The dominant wear mechanism was abrasive wear and slight adhesive wear for the non-texture UHMWPE sample, and the wear mechanism of textured samples was slight abrasive wear. The dominant wear mechanism was abrasive wear and slight adhesive wear for the non-texture UHMWPE sample, and the wear mechanism of textured samples was slight abrasive wear. • The wear performance of the UHMWPE with composite texture was superior to that of other textures, which was mainly attributed to the micro hydrodynamic bearing effect and stepped bearing effect. [ABSTRACT FROM AUTHOR]

Abstract Friction force of the linear motion ball guide (LMBG) with preload is important to predict the mechanical efficiency, thermal deformation, and stiffness variation in optimal design processes of the precision machinery. In this paper, an explicit friction force model of the LMBG with a two-point ball contact structure has been formulated in terms of the material and geometric parameters as well as the assembly and operating conditions. Contrary to the LMBG maker's model with accuracy 63%, the prediction accuracy over 90% is verified through experiments. It is confirmed that the friction force depends upon the ball size, conformity factors of the grooves, lubricant viscosity, and ball contact angles. Sliding and viscous frictions are significantly influenced by operating conditions as well. Highlights • Hysteresis, sliding and viscous friction components are formulated explicitly. • Friction force model is a function of geometric, assembly and operating conditions. • The sliding and viscous frictions are dominant at low and high speeds. • Precision measurements confirm the accuracy of the formulated friction model. • High efficient LM guides with good stiffness are designed easily via the model. [ABSTRACT FROM AUTHOR]

This paper optimizes the coverage of circular micro-textures on bearing sliders aiming at improving their tribological performance. Grid-like array of textures are arranged to present or absent, which are respectively marked as 1 and 0. Genetic Algorithm is used to evolve the coverages of textures. The algorithm allows the derivation of preferable area for texture coverage in shape of pyramid at the inlet of slider. Evaluation of influences of convergence ratio and width demonstrates that sliders with small convergence ratio and width tend to result in significant tribological improvement effect. Subsequent experimental tests confirmed the achievement of low friction coefficient of the so optimized texture coverage. [ABSTRACT FROM AUTHOR]

This study proposes an approximate analytical model on frictional dynamics of a single visco-elastic pillar used for texturing soft surfaces. The frictional dynamics and contact interaction of the pillar with the smooth flat counter surface is modeled with lumped parameters in order to estimate the friction force and the normal load. The effect of aspect ratio of the pillar, preload and sliding velocity are obtained numerically and the analytical model is validated experimentally. The relative error of the proposed model on the calculation of friction force is around 5% while preload applied is in the neighborhood of Euler buckling limit for aspect ratio of 4, and increases to 30% at lower preload values for aspect ratio of 2. [ABSTRACT FROM AUTHOR]

Measuring friction on the human body is challenging on areas where the surface of the skin is curved. Handheld devices for measuring friction may make it easier to rapidly measure from hard-to-reach areas, however, the orientation of the probe relative to the surface is often sub-optimal to reliably measure the normal force. Here, the friction on the surfaces of a mannequin and a human body are measured and corrected using a curvature correction technique. This is accomplished by accounting for the shape of the measurement surface and the orientation of the measurement device relative to that surface. Two strategies for constructing the surface shape of the measurement surface are compared. [Display omitted] • Implement a curved surface correction method in 3D with a linear sliding handheld tribometer. • Test methods for capturing the 3D curved surface shape for the surface correction method. • Measure sliding friction on a human body. [ABSTRACT FROM AUTHOR]

The aim of this work was to determine the mechanism of ultra-low friction between a self-mated Si 3 N 4 pair lubricated with water after running-in. The key to solving this problem was to determine the lubrication regime of the interface under ultra-low friction conditions. According to the Stribeck curve, the variations of friction coefficient with load and velocity were measured to evaluate the hydrodynamic effect; and the variation under different lubricating oils was examined to investigate the influence of viscosity and eliminate the influence of any tribochemical reaction between Si 3 N 4 and water. Surface wear after a long-duration friction test under ultra-low friction conditions was also evaluated. It was found that the ultra-low friction mechanism was attributed to a pure hydrodynamic effect. [ABSTRACT FROM AUTHOR]

Frictional behaviour of mixed Elasto-hydrodynamic lubrication (EHL) arises from the coupling of the lubricant fluid and asperity interaction frictions. Due to the difficulties in modelling asperity interaction friction, existing approaches often use a constant friction coefficient obtained in boundary lubrication to approach the asperity interaction friction coefficient in mixed EHL. In this study, the asperity interaction friction is, for the first time, considered to result from either the boundary film friction or solid-to-solid ploughing and adhesion friction depending on the local contact and deformation conditions. The friction of mixed EHL was predicted by the combination result of lubricants fluid, boundary film, and solid-to-solid frictions. This development provides an alternative and cost effective method to estimate friction coefficient in mixed EHL. [ABSTRACT FROM AUTHOR]

Tribological conditions in bulk metal forming processes are characterized by high contact pressures and large relative movements at high velocities between the tool and the workpiece. In addition, significant surface enlargements and elevated temperatures can occur. It is very challenging to reproduce such contact conditions on pin-on-disc tribometers. In this study, a method for simulating the contact conditions of a metal forming process on a pin-on disc tribometer is presented. Special test samples have been designed in order to achieve high contact pressures and surface enlargements with a homogeneous distribution of these quantities within the contact zone. It is shown that the test method delivers reproducible results and enables the detection of small changes of the contact conditions. Furthermore, microstructural changes of the real contact surface can also be simulated, which was verified by SEM imaging. [ABSTRACT FROM AUTHOR]

A novel tribometer for the measurement of friction torque in silicon-based microball bearings is demonstrated. First, a new configuration of microball bearing is proposed based on the contact mechanics analysis. The bearing is fabricated using halo etch and intermediate layer bond processing methods and the friction torque test platform is then developed. It is found that the microscale rolling friction torque between microball and raceway exhibits a sublinear relationship with load, which is different from the case of macroscale rolling, and the Si 3 N 4 ball bearings generate larger friction torque than 440C ball bearings under the same load. [ABSTRACT FROM AUTHOR]

A multi-objective optimization approach is used to determine the optimal surface texturing parameters to improve performance of journal bearing. The numerical attempts are made using Taguchi's orthogonal array L 27 . Initially, static performance analysis is carried out for plain bearing system using thin film analysis by COMSOL Multiphysics and the results are validated with the literature. In the later part, analysis of load carrying capacity and frictional torque is carried out for different parameters of surface texturing. The bearing performance is studied for various groove parameters viz.groove location, groove height, groove width, number of grooves and spacing between grooves. Grey relational analysis is used to find an optimal set of parameters which gives maximum load carrying capacity and minimum frictional torque. [ABSTRACT FROM AUTHOR]

Surface, as the fingerprint of a component, is of interest to reveal the nature of sliding wear. Many researchers attempted to link the coefficient of friction (COF) to the surface topography from the perspective of a classical single-stratum surface, but few studies have focused on a bi-Gaussian stratified viewpoint, whereby wear mechanism can be understood. Herein, a surface-surface dry sliding experiment with silicon carbide versus carbon graphite is discretely stopped, cleaned, measured and restarted to obtain the histories of the bi-Gaussian surface parameters and the COF during the wear process. Two bi-Gaussian evolution modes are observed, from the surface-topography viewpoint, providing a reasonable explanation why the COF remains unchanged during the stable period wherein the surface topography varies with time. [ABSTRACT FROM AUTHOR]

The performance of carbon nitride ( CN x ) coated roller bearings is investigated, using a micropitting rig. The rolling performance is evaluated using Stribeck test, with a continuously varying rolling speed (0.2 – 2 m/s). Rolling contact fatigue tests with constant speeds (0.5, 1, 2, and 3.5 m/s) are also conducted in order to study the high-cycle performance of the rollers. The obtained Stribeck curve shows that the presence of coatings eliminates run-in, resulting in low friction coefficients ( ∼ 0.08 ). Raman spectroscopy, performed at the wear tracks, reveals that CN x maintain stable chemical state. Coatings show abrasion although the wear rate is not detrimental for the performance of the rollers, since a CN x -to-steel contact is retained during the entire rolling contact fatigue test. [ABSTRACT FROM AUTHOR]

The paper describes the tests carried out to determine the applicability of a simple means for reducing friction force in linear pneumatic actuators. The idea is based on the use of commercial lip seals mounted in the direction opposite to that used in conventional systems. Two different non-conventional installations were tested using commercial spring-energized seals consisting of two different materials: virgin PTFE and graphite-filled PTFE. Tests assessed the effect of bore manufacturing tolerances with nominal bore diameters of 50.0 mm and 50.2 mm. Performance was evaluated in terms of leakage flow rate and friction force, comparing the proposed installation with conventionally mounted seals. Friction coefficient and stiffness of the two seal materials were measured on a seal segment mounted on a pin-on-disk tribometer featuring a special pin design. Tests were carried out by loading the seal segment against a portion of the cylinder barrel in reciprocating motion. Stiffness and friction coefficient measurements served as the basis for developing a preliminary lumped parameter model of the lip seal which was used to analyze lip deflections, exchanged forces and leakage flow rate between seal and barrel. • Non-conventionally mounted lip seals. • Leakage flow rate and friction force versus bore diameter and supply pressure, were measured. • Tribometer with a purpose-built seal carrier pin was used to measure seal friction coefficient and the elastic stiffness. • Simplified lumped parameter model of the seal: contact forces and leakage flow rate were determined. [ABSTRACT FROM AUTHOR]

Predicting tyre–road friction requires various inputs that are known with differing levels of confidence. This paper studies the prediction and associated experimental confirmation of rubber friction on real roads at high sliding speeds. Friction predictions are obtained from Persson's flash temperature model: the topography of the road surface is measured using an optical profilometer, while the rubber's viscoelastic modulus is obtained through Dynamic Mechanical Analysis. A newly developed friction tester performs in-situ friction measurements, while controlling and monitoring bulk and contact surface temperature, respectively. Local topographical road roughness variations were identified as a major contributing factor leading to predicted friction variations of over 50%, while the flash temperature predictions showed good correlation with temperature measurements from near the rubber–road interface. • Rubber friction testing with flash temperature measurement under real conditions. • Persson's flash temperature model without adhesion showed agreement with experiment. • Rubber background temperature is less important in practical situations. • Partial validation of flash temperature predictions. [ABSTRACT FROM AUTHOR]

Atomic force microscopes could be used in wide range of nanotribology experiments but probes available on the market are only made of silicon or silicon nitride with a stiffness in the range of 0.01–100 N/m, which significantly limits the possible research. We strive to solve this problem by designing all-metal probes. The proposed fabrication method is characterised by the use of a copper substrate and electrodeposition in a mould prepared by indentation and photolithography. Prototype probes fabricated with this method were made of nickel with a stiffness of 20 N/m and 2800 N/m and were used for topography and friction measurements. Both the method and all-metal probes showed flexibility and great potential, especially in the field of nano/microtribology. [ABSTRACT FROM AUTHOR]

This study conducted friction experiments of three elastomer block materials on ice (low pressure injection of polyurethane (PU), injection moulding of a thermoplastic polyurethane (TPU) and vulcanization of rubber (RU)), with a linear tribometer operating under four different ambient air temperatures (−10 to 0 °C) and six different sliding velocities (0.3–5 m/s). At low temperatures (−10 °C and −5 °C), the RU material showed the highest dynamic and static friction. However, at high temperatures (−2 °C and 0 °C) the PU revealed the highest dynamic friction and highest static friction at 0 °C. We discuss the physical phenomena's of elastomer friction on ice and in relation to the application of footwear slipping on ice. • Slipping accidents simulated by biofidelic friction testing of elastomers on ice. • PU outsole material on warm ice provides higher static friction coefficient compared to TPU and RU materials. • RU outsole material revealed higher dynamic friction coefficient on cold (−10 to 5 °C) ice compared to TPU and PU materials. • Hybrid outsole constructions combining materials, composites and design solutions may be able to accommodate slip resistance. [ABSTRACT FROM AUTHOR]

The tribological interaction between patterned running surfaces and an ice track was investigated. The friction coefficient was analyzed according to the patterned running surface type, ambient temperature, and rotational track velocity. The results were analyzed based on the structural characteristics of the patterned running surface in terms of average height and skewness. Generation of the water layer was visualized using transparent replicas of the patterned running surfaces. The water layer ratio was then calculated and compared with the friction coefficient. We found that the water layer either functioned as a lubricant or offered frictional resistance depending on the water layer ratio and the type of patterned running surfaces as the lubricating friction regime changed. • The friction force between the commercially available patterned running surfaces of jumping ski and ice is measured. • The water layer is visualized to elucidate the effect of the surface patterns on ice friction coefficient. • The lubricating friction regime varies by surface patterns , resulting either increases or decreases frictional drag. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS 2) is a two-dimensional material that exhibits unique interfacial interactions with gold (Au) and silicon (Si). These interfaces have gained attention due to their potential to be integrated into a broad range of applications, where they involve direct contact that affects their performance. Here we investigate the nanoscale mechanical contact interaction of MoS 2 monolayers with Au and Si, using adhesion measurements and friction force microscopy. MoS 2 -Au contact exhibits pronounced adhesive interaction, manifested by higher adhesion pull-off forces and energies, compared to MoS 2 -Si. Influenced by the adhesive contacts, the friction forces recorded demonstrate stronger lateral interactions and stronger interfacial shear strength between MoS 2 and Au. Analysis with the Prandtl–Tomlinson model reveals that the MoS 2 -Au direct interaction is also stiffer, suggesting higher elastic energy at the contact. This information on the mechanistic nature of MoS 2 -Au and MoS 2 -Si contacts under shear and normal loads is of potential importance in applications where such interfaces are utilized, such as in the design of coatings, sensors and mechanoelectrical devices. • MoS 2 is a 2D that exhibits unique interactions with gold and silicon. • Nanoscale contact mechanics of MoS 2 interfaces with Au and Si are investigated. • MoS 2 -Au contact exhibits pronounced adhesive interaction compared to MoS 2 -Si. • Friction at MoS 2 -Au contact is stronger with larger interfacial shear-strength. • Stiffer lateral MoS 2 -Au interaction suggests higher elastic energy at the contact. [ABSTRACT FROM AUTHOR]

In situ measurement of frictional properties, particularly of human skin, is challenging. A major challenge is that the measured surface may be curved and oriented arbitrarily relative to the measurement device, making it difficult to obtain accurate and reliable friction estimates. Here we propose a method to simultaneously estimate the orientation of the surface during friction measurements, using a custom-made hand-held tribometer. The method accounts for the position and orientation of the tribometer relative to the shape of the measured surface. Our simple calibration method allows lightweight tribometers to be built with cheaper materials like plastic, without sacrificing accuracy. [Display omitted] • Propose a general method to measure force data on curved surfaces for sliding contacts and friction analysis. • Implement this curve correction method with a linear sliding handheld tribometer. • Describe the importance of carefully testing and calibrating the outputs of lightweight devices that may have low flexural rigidity. [ABSTRACT FROM AUTHOR]

Atomic-scale friction measurements were performed on a metallic glass in corrosive solutions using an atomic force microscope. An irregular stick-slip motion was observed and attributed to the amorphous structure of corroded surfaces. The friction data are interpreted based on the Prandtl-Tomlinson model by considering the disordered surface interaction potential. The transition between different stick-slip regimes, namely smooth sliding, slips over a basic lattice distance or multiple lattice sites, is a position-dependent characteristic on the amorphous surface. The stronger corrosion causes a significant increase in the probability of multiple slips accompanied by larger maximal forces and leads to a higher average friction. Our findings contribute to the understanding of atomic-scale friction on amorphous surfaces and fundamental friction mechanisms in corrosive conditions. • Irregular stick-slip friction is observed by AFM on a metallic glass in corrosive solutions. • The effect of corrosion on friction is explained by frictional slip over multiple lattice sites. • An analytical model is proposed to interpret nanoscale stick-slip friction on amorphous surfaces. [ABSTRACT FROM AUTHOR]

The effect of water menisci at the asperity junction on the friction level and oscillation was investigated by using atomic force microscopy. The menisci formed through capillary action were controlled by changing the humidity of the experimental environment and the ZnO substrate's hydrophilicity. The hydrophilic ZnO substrate showed higher friction levels than the hydrophobic substrate, while this difference was indistinguishable at humidity levels above 40% relative humidity. Indentation experiments demonstrated that the friction level was determined by the stiffness of the water menisci that was changed by hydrophilicity of the substrate and humidity level. The stick-slip amplitude was also increased in the case of the hydrophilic substrate, supporting the hypothesis of repeated formation and deformation of the solid-like menisci during sliding. [ABSTRACT FROM AUTHOR]

A low degree of freedom, semi-analytical model for rapid estimation of the friction coefficient in elastohydrodynamically lubricated contacts was developed and tested. Its estimates are based on the shear rate dependent Carreau equation for the apparent viscosity, together with the hydrodynamic pressure and the temperature of the lubricant. To validate the approach, the model׳s predictions were compared to experimental coefficient of friction measurements acquired using a ball-on-disc test device at various applied loads, entrainment velocities, and slide to roll ratios. The model׳s predictions were in good agreement with the experimental results, showing that it is suitable for use in multibody dynamics analyses where rapid computation of elastohydrodynamic friction is required to minimize computing time and resource consumption. [ABSTRACT FROM AUTHOR]

The influence of molecular structure of organic friction modifiers (OFMs) on their adsorption behaviour on iron oxide surface immersed in PAO oil was investigated using neutron reflectometry (NR). The effects of carbon chain length and levels of unsaturation on film formation was studied using a series of fatty acids with 18–20 carbon atoms in the temperature range between 25 and 100 °C. The results revealed that saturated OFM molecules generate thicker films compared to its unsaturated counterparts. Further, a decrease in film thickness was observed with the degree of unsaturation with the exception of linolenic acid. The film thickness and grafting density of the OFM increased with temperature. A decreased film thickness for the binary additive system comprising GMO and saturated molecules suggested that the adsorption of OFMs on the surface is hindered in the presence of glycerol mono oleate (GMO). A lower boundary friction coefficient and wear exhibited by OFMs with longer chain and linear structure suggest that their tribofilms are more effective in withstanding shear. The tribofilm thickness measured using optical interferometry showed no correlation with the adsorbed layer thickness determined from NR data; this lack of correspondence is attributed to the load and shearing forces experienced by the OFM molecules at the contact surfaces during friction measurements. • Film thickness decreased with levels of unsaturation with the exception for linolenic acid. • Lower boundary friction coefficient and wear was exhibited by OFMs with longer chain and linear structure. • No correspondence between film thickness from optical interferometry and neutron reflectometry. [ABSTRACT FROM AUTHOR]

Abstract: The developers of innovative automotive active systems have recently stimulated new interest toward the analysis of the frictional behaviour of brake and clutch facings. This paper presents the experimental results acquired with a laboratory setup on brake and clutch facing samples in sliding motion for different operating conditions. An artificial neural network has been used to obtain a comprehensive view of the influence of the main sliding parameters. The study has also taken into account the not weak influence of the sliding acceleration to improve the friction coefficient prediction during transient operations of these dry friction based devices. [Copyright &y& Elsevier]

Finger pad friction varies significantly between individuals due to personal differences and environmental effects. For this reason, comparison of the absolute friction values becomes non-informative and is rarely reported in literature. This work investigates whether friction data can be normalised to correlate between individuals. A set of textured samples were tested by 10 volunteers in dry sliding conditions with a range of normal loads up to 4 N. A reference smooth sample was used for each participant to normalise the dataset. The proposed approach allows to eliminate one of the unknown parameters, e.g., finger pad ridge contact area function, and discuss the data with respect to other variables. • Friction measurements from 10 volunteers, 4 samples and 9 normal loads were analysed to propose a normalisation method. • Normalisation reduces one of the personal variables and allows friction comparison between individuals. • Friction measurements normalised by a reference represent a change of contact area fraction with normal load. • Volunteers showed similar fractions of real contact area despite differences in skin properties and friction. [ABSTRACT FROM AUTHOR]

Advances in internal combustion engines make it necessary to study in depth their contribution to engine efficiency, and thus to friction mechanical losses, especially those generated in the piston-cylinder liner assembly. In this paper, the design of a novel test rig is presented, based on the floating liner principle, with the key objective of having the capability of developing multiple parameters tests in a relatively quick and easy way. The final design integrates a single-cylinder internal combustion engine, a mechanical solution that allows the liner to float, three piezoelectric force sensors and a novel design of lateral support based on ball transfer units. Reproducibility and repeatability of tests were assessed demonstrating the capability of the test rig to develop reliable friction measurements. Some parameters tests were developed to evaluate the sensitivity of the friction force measurements to variations in the working conditions, oil temperature and engine speed. A theoretical model for the piston compression ring is presented for both fully flooded and starved lubrication conditions, including asperity interactions and lubricant rheology. Comparisons of these approaches with experimental results showed that, for the working conditions of the test rig, lubrication of the compression ring is better estimated under fully flooded lubrication condition, as the starved lubrication analysis results in over-estimated friction force values at higher engine speed regimes. • Design and validation of a floating liner for measurement of piston friction force. • Experimental compression ring friction as function of speed and oil temperature. • Piston compression ring model for fully flooded and starved lubrication conditions. • Comparison of experimental and theoretical results for the piston compression ring. • Piston compression ring working mainly under fully flooded lubrication conditions. [ABSTRACT FROM AUTHOR]

Investigation of friction force induced by seals is important in improving control accuracy and evaluating sealing performance for hydraulic actuators. An experimental apparatus was developed to measure the friction forces for hydraulic cylinders under different operating conditions based on position and chamber pressures measurements only. The effects of velocity, external load, seal diameter, and sealing profiles on steady state friction force of hydraulic cylinders were examined. In addition, the influences of amplitudes of velocity and acceleration on dynamic friction force were also highlighted. It was shown that the hydraulic actuator with combined piston seal demonstrates the highest friction force and is less sensitive to variation of load than that of Glyd ring and U-cup seal. • A friction force test method is proposed for hydraulic actuators using position and chamber pressures measurements only. • Friction behaviors of hydraulic actuators with different sealing profiles have been experimentally investigated. • A novel combined piston seal is evaluated in terms of friction measurement and is compared with commonly used seals. • The effects of velocity, load, seal diameter, seal cross section, and acceleration on friction behaviors are studied. [ABSTRACT FROM AUTHOR]

A critical appraisal of the Shoe Slip Tester, SST, is discussed the paper. The SST is designed to be a portable and practical alternative to the Ramp tester which is currently considered the Gold Standard in slip resistance testing. Tests identical safety shoe samples were carried out using the SST and RAMP in parallel and the data yielded compared. Data showed that the Shoe Slip Tester yielded higher coefficients of friction. Controlling the data for force applied at shoe impact gave good correlation between the two devices. Recommendations are made to the design of the SST from data in this paper. Recommendations will improve correlation between the two devices. The SST can be said to be a suitable alternative. • A new device which can be used for the research and development of slip resistant footwear. • The device shows very good correlation when used by different users in different locations. • This is a new device which allows the direct measurement of friction between a specific shoe and floor combination. • Comparison has been made between this device and others such as the RAMP tester. • Good correlation is seen between this new device and the RAMP tester. [ABSTRACT FROM AUTHOR]

Experiments were performed to investigate the effect of micropitting on friction coefficient. Two twin-disc machines were used: one to develop micropitting on a disc pair, the other one to measure the friction coefficient. Fatigue was endured on one disc pair; another was used as a reference to dissociate the impact of micropitting from other factors. The study shows that friction coefficient was impacted by the development of micropitting. It also underlines a clear correlation between micropitting magnitude and friction coefficient. The influence on friction of classical parameters, such as roughness and oil, was also discussed in order to validate that only micropitting growth influenced friction measurements. • The impact of micropitting on the friction coefficient of two lubricated surfaces is investigated. • Two twin disc machines are used: one to generate micropitting, one to measure friction coefficient. • A correlation between micropitting magnitude and friction coefficient increase is underlined. [ABSTRACT FROM AUTHOR]